The invention relates to a direct printing apparatus and method in which a computer generated image information is converted into a pattern of electrostatic fields, which selectively transport electrically charged particles from a particle carrier towards an image receiving surface through a printhead structure, whereby the charged particles are deposited in image configuration on the image receiving surface caused to move relative to the printhead structure.
More particularly, the invention relates to improve the print quality and sharpness of the printed characters in direct printing.
U.S. Pat. No. 5,036,341 discloses a direct electrostatic printing device and a method to produce text and pictures with toner particles on an image receiving substrate directly from computer generated signals. Such a device generally includes a printhead structure provided with a plurality of apertures through which toner particles are selectively transported from a particle source to an image receiving medium due to control in accordance with an image information.
The printhead structure according to U.S. Pat. No. 5,036,341 is formed by a lattice consisting in intersecting wires disposed in rows and columns. Each wire is connected to an individual voltage source. Initially the wires are grounded to prevent toner from passing through the wire mesh. As a desired print location on the image receiving substrate passes below an intersection, adjacent wires in a corresponding column and row are set to a print potential to produce an electric field that draws the toner particles from the particle source. The toner particles are propelled through the square aperture formed by four crossed wires and deposited on the image receiving substrate in the desired pattern. A drawback with this construction of printhead structure is that individual wires can be sensitive to the opening and closing of adjacent apertures, resulting in imprecise image formation due to the narrow wire border between apertures.
This effect is mitigated in an arrangement described in U.S. Pat. No. 5,847,733 by the present applicant. This proposes a control electrode array formed on an apertured insulating substrate. A ring electrode is associated with each aperture and is driven to control the opening and closing of the aperture to toner particles. Each aperture is further provided with deflection electrodes. These are controlled to selectively generate an asymmetric electric field around the aperture, causing toner particles to be deflected prior to their deposition on the image-receiving member. This process is referred to as dot deflection control (DDC). This enables each individual aperture to address several dot positions. The print addressability is thus increased without the need for densely spaced apertures.
Although the print quality in direct electrostatic printing is very good there is a continuous need for further improvements.
Accordingly, the object of the present invention is to improve the print quality and sharpness of the printed characters in a direct electrostatic printing apparatus. In accordance with claim 1, this object is achieved by means of control means that is arranged to address an amount of charged toner particles corresponding to at least 35 charged toner particles per addressed dot at least for a part of the image location areas. It is by that ensured the number of toner particles in the respective dot is sufficient for providing a distinct edge sharpness in that dot.
Further characteristic features and advantages of the invention are disclosed in the description and in the dependant claims.